P2P Economics, BitTorrent and Kademlia

Indicate the correct statements regarding free riders in P2P systems (2 correct answers). A free rider benefits from other peers without contributing resources. Free riders increase the total system throughput. Studies showed that most query responses come from a very small fraction of peers. Free riders are always blocked by BitTorrent trackers. Free riding is impossible in decentralized systems.

Which statements correctly describe the Tragedy of the Commons? (2 correct answers). It occurs when individuals overuse a shared resource. It only applies to physical resources like land. It is caused by positive externalities. The resource is non-excludable. It guarantees fair sharing.

Which statements about the Prisoner’s Dilemma are correct? (2 correct answers). Defection is the dominant strategy. Mutual cooperation gives the worst outcome. The total outcome is higher if both cooperate. Rational peers will always cooperate. T < R < P < S.

Why are incentive mechanisms needed in P2P systems? (2 correct answers). To prevent selfish behavior. To guarantee anonymity. To avoid performance degradation. To eliminate trackers. To make encryption unnecessary.

Which are properties of barter-based incentive systems? (2 correct answers). Immediate bilateral exchange. Multilateral trading. Low transaction costs. Vulnerable to double spending. Use of virtual currency.

Which are characteristics of bond-based systems? (2 correct answers). ab. ab.

What is BitTorrent mainly designed for? (1 correct answer). Real-time streaming. Distributed computation. Efficient replication of large static data. Encrypted messaging. Decentralized search.

Which elements are stored in a .torrent file? (3 correct answers). ab. ab.

What are correct statements about a BitTorrent tracker? (2 correct answers). It stores the file data. It coordinates the swarm. It returns random peer IPs. It enforces encryption. It selects rarest pieces.

Which are benefits of swarming? (2 correct answers). Multi-source download. Centralized bandwidth control. Tolerance to peer failures. Removal of hashing. No need for incentives.

What does “choking” mean in BitTorrent? (1 correct answer). Refusing to upload to a peer. Disconnecting from the tracker. Losing a piece. Stopping downloads. Leaving the swarm.

Which rules belong to the unchoking algorithm? (2 correct answers). Unchoke fastest uploaders. Always unchoke everyone. Use optimistic unchoking. Select peers randomly every time. Use rarest-first.

Why is optimistic unchoking used? (1 correct answer). To punish free riders. To allow new peers to enter. To reduce hashing cost. To improve security. To balance piece overlap.

High piece overlap causes: (2 correct answers). Fewer trading opportunities. Better bandwidth utilization. Underutilized bandwidth. Faster downloads. More fairness.

What does a low number of distributed copies imply? (1 correct answer). High redundancy. Danger of piece loss. Perfect balance. Higher throughput. No failures.

Why does BitTorrent use rarest-first? (2 correct answers). ab. ab.

When is Random First used? (1 correct answer). Always. At the end. At the beginning. Only by seeders. When tracker fails.

What is the purpose of endgame mode? (2 correct answers). Request remaining blocks from all peers. Reduce bandwidth usage. Avoid waiting for rare blocks. Stop the download. Disable choking.

What does the Coupon Collector problem describe? (1 correct answer). Hash collisions. Difficulty of getting last missing items. Tracker overload. Encryption cost. Network flooding.

Which are theoretical attacks on BitTorrent? (3 correct answers). Sybil. Collusion. Whitewashing. Flooding. Finger poisoning.

Which actions are used by BitThief? (2 correct answers). Contact tracker frequently. Upload rarest pieces. Exploit optimistic unchoking. Always choke others. Use encryption.

Which properties are correct? (2 correct answers). It is symmetric. It is based on subtraction. d(A,B)=d(B,A). It uses Euclidean distance. It is asymmetric.

How does Kademlia store data? (2 correct answers). On k closest nodes. On random peers. Using XOR distance. On the tracker. Only on the origin peer.

What is α used for? (1 correct answer). Encryption. Parallel lookups. Hashing. Replication. Routing table size.

Which are valid Kademlia RPCs? (3 correct answers). FIND_NODE. STORE. FIND_VALUE. SEARCH. PUSH.

Which are true? (2 correct answers). Routing tables differ between peers. Each bucket covers a distance range. All peers have identical tables. They store file hashes. They store content blocks.

Which rule is correct? (1 correct answer). Always replace the oldest node. Keep the least recently seen node if alive. Never split buckets. Only store one peer per bucket. Delete all entries.

Why are old contacts preferred? (1 correct answer). They are faster. They are more likely to stay online. They store more data. They use less bandwidth. They encrypt traffic.

Which are Kademlia enhancements? (2 correct answers). Parallel queries. Passive learning. Central server. Fixed routing tables. No replication.

Which statement is correct? (1 correct answer). BitTorrent is a DHT. Kademlia is an incentive system. BitTorrent uses tit-for-tat. Kademlia uses trackers. Both are flooding-based.